This invention relates to electrical resistance ceramic heating elements and is particularly, although not exclusively, applicable to silicon carbide electrical heating elements.
Electrical resistance heating is a well-known process. Electricity is passed through a resistive element that generates heat in accordance with well-known electrical laws. One group of electrical resistance heating elements comprises silicon carbide rods that have an electrical resistance varying along their length. In these elements the majority of heat generated is in high resistance parts referred to as the xe2x80x9chot zonexe2x80x9d, lower resistance parts where less heat is generated being referred to as xe2x80x9ccold endsxe2x80x9d. The rods conventionally are solid rods, tubular rods, or helical cut tubular rods. The purpose of helical cutting a tubular rod is to increase the length of the electrical pathway through the hot zone, and reduce the cross-sectional area of the conductive path, and so increase the electrical resistance. Typical rods of this type are Crusilite(trademark) Type X elements and Globar(trademark) SG rods. Helical cut tubular rods of this nature have been known for at least forty years.
In such a tubular rod electrical connections are made at cold ends either side of the hot zone. For some purposes it is desired to have the electrical terminals at one end. Accordingly for at least 30 years it has been known to provide a tubular rod having a double helix, one end of the rod being split to provide cold end electrical terminals and the other end providing a junction between the two helixes. Typical elements of this type are the Crusilite(trademark) DS elements and Globar(trademark)SGR or SR elements.
The current practice for Crusilite(trademark) elements (X, MF, DS and DM) is to cut the helical groove into the silicon carbide tube using a diamond wheel. The pitch of the helix depends upon the resistance of the silicon carbide tube and the required resistance of the Crusilite(trademark) element. The tighter the pitch, the higher the resistance obtained from a given tube. For a double helical element (DS or DM), two helical cuts are made, starting at 180xc2x0 to each other and with the second helix mid-way between turns of the first helix. The helix is then extended at one end by slitting with a diamond saw, the slit end becoming the terminal end for the electrical connections.
For manufacture of the Globar(trademark) helical element (SG, SGR), the helix is cut into the tube using a diamond drill before firing. For the double helix element (SGR) two cuts at 180xc2x0 to each other are used. After cutting the helixes, the material is fired in a 2-stage process during which the final resistance is controlled.
All of these elements (Crusilite(trademark) X, MF, DS, DM, Globar(trademark) SG, SGR) are single-phase elements and are used in a wide range of both industrial and laboratory furnaces operating, for example, at temperatures between 1000xc2x0 C. and 1600xc2x0 C.
Where high levels of heating are required and the number of heater units is a multiple of three it is frequently the case that a three-phase power supply is used. It is desirable that the power in each of the three phases is the same and, for that reason, single-phase elements are normally installed in multiples of three. Alternatively, three-phase silicon carbide elements can be used, ensuring a balanced three-phase load in cases where the number of elements installed is not divisible by three. Conventionally silicon carbide three-phase electric elements consist of three legs bonded into a common bridge. The legs are normally either arranged in a plane (so the element has the appearance of cricket stumps), or arranged in a triangle (in a format sometimes referred to as a milk stool format or as a Tri-U). The cricket stump arrangement has been known since at least 1957 (see GB 845496) and the Tri-U arrangement since at least 1969. Manufacturing such elements conventionally requires separate manufacture of the legs of the element and then bonding to a bridge. It has in the past been proposed to manufacture such elements by casting in one piece but one-piece elements are not common in the market place. It has also been proposed to combine three-helically cut elements to a common bridge in cricket stump type arrangement (see GB 1279478).
It is known to combine pairs of elements in a generally U-shaped configuration so that the terminals of the elements are at one end. A typical such element is the Kanthal Type U element. (For other U-shaped elements see for example GB 838917 and U.S. Pat. No. 3,964,943). Several of these elements may be required for a given heating application. For applications where there are confined spaces it can be extremely complex to provide suitable arrangements to connect the elements to an electrical supply. Further, many holes need to be provided for the power supply to these elements. These holes can threaten the structural integrity of the thermal insulation of a heating appliance and in addition are detrimental to thermal efficiency as heat may pass out of the furnace through the holes or along the conductors. An arrangement that has been proposed is that of GB 1123606 which discloses a so-called xe2x80x9csquirrel cagexe2x80x9d arrangement of bar elements mounted in and spaced apart by refractory rings and interlinked by screw connection to bridging conductors. This arrangement is complex and includes many electrical interconnections.